1. Field of the Invention
The invention relates to a scanning system for three-dimensional imaging, and more particularly to the scanning system of computed tomographic imaging that choose the most appropriate scan mode based on the characteristics of the subject to be scanned so as to obtain a corresponding optimal image by analyzing the transmitted projection data within a limited scan range.
2. Description of the Prior Art
Generally, conventional transmission imaging methods include the 2D plane radiography (such as an anteroposterior or a posteroanterior chest X-ray), the 3D computed tomography (CT), and so on. By having the X-ray imaging as an example, the 2D plane radiography can only provide a planar image without in-depth and clear information of individual organs. For instance, in a chest X-ray imaging, lesions at the cardiac area, mediastinum, diaphragm, spine and so on are not easy to be seen. On the other hand, though the CT can provide images of different depths, yet the cost is about ten times of the X-ray radiography and the radiation dose would be up to a hundred times of the X-ray radiography. Hence, the CT is hard to be the first-line diagnostic choice. According to the report No. 160 of the NCRP (National Council on Radiation Protection), the average yearly effective dose of the people in the United States is raised from 3.1 mSv in 1980 to 5.5 mSv in 2006. It is noted that the major reason responsive for such a hike is the increase of the radiation for medical examinations and therapies (from 0.5 mSv to 3.0 mSv), mainly from the CT. Thus, the issue of minimizing the risk and dose of the radiation without sacrificing medical image quality is one of the mainstream topics of the radiologic researches.
Besides the aforesaid 2D plane radiography and CT, one of recent developments is a limited-angle scan imaging method, called as the digital tomosynthesis. However, because only the projection information from limited-angle range can be provided, we believe that an appropriate scanning direction should be applied to different body portions. It is also understood that the spatial resolution of the tomosynthesis is directionally dependent, and the choice of the scanning direction therefor is sensitive and thus significant to the subject to be scanned. However, a conventional general purpose X-ray apparatus with the tomosynthesis function can perform the scanning only along a longitudinal direction. Definitely, such a uni-directional scanning pattern upon the subject to be scanned can't meet various demands anymore for a universal X-ray apparatus.
Generally, there are three types of radiographies: 2D radiography, dynamic fluoroscopic radiography, and computed tomography. For a general purpose X-ray apparatus, the first two types of radiographies are the basic, but the last one is optional. Recently, for the development in the digital image detector becomes mature, major manufacturers such GE and Shimadzu have constructed the apparatus with the tomosynthesis functions. Contrary to the omni-angle scanning pattern of the conventional tomography, the new digital tomosynthesis can only capture the projection information within a limited range. For the direction of the scan trace can affect the imaging result, a current radiographic apparatus with the tomosynthesis function can only provide single-direction scanning, typically the longitudinal direction. Yet, it is understood that such a uni-directional scanning can't meet the demand of subjects with different characteristics to be scanned.
In U.S. Pat. No. 6,632,019, the radiographic apparatus has an X-ray tube device fixed to a pole and an X-ray image detecting portion facing the X-ray tube device to sandwich a top board therebetween so that fluoroscopying or radiographing can be carried out. It is noted that, while in scanning a lied-down subject, the X-ray tube as well as the X-ray image detecting portion are synchronously moved parallel with respect to the top board along an axial direction (i.e. the longitudinal direction of the top board). While the subject to be scanned in an upright position, the aforesaid X-ray tube device is individually evacuated toward a head or feet side of the subject, and a secondary X-ray tube device suspended from a ceiling comes in for performing fluoroscopic radiographing upon the subject in cooperation with the X-ray image detecting portion. In this disclosure, the support for the X-ray image detecting portion can be manually moved along, and only along, the longitudinal direction of the top board. No matter what position mode of the subject is, the apparatus of U.S. Pat. No. 6,632,019 provides only a longitudinal scan direction.
Further, in U.S. Pat. No. 5,734,694, another universal radiographic apparatus is disclosed. This radiographic apparatus allows an operator to select between a conventional radiographic mode and a linear tomographic mode. While in the conventional radiographic mode, an X-ray tube mounted to a tube crane positioned above an elevating table can be accurately controlled in longitudinal and vertical movement. Also, in the conventional radiographic mode, the apparatus may include following three operational patterns: (1) the table bucky automatically tracking motion of the tube crane, (2) the tube crane tracking vertical motion of the table so as to maintain a fixed SID, and (3) the tube crane tracking vertical movement of an associated wall bucky. While the audiographic apparatus is in the linear tomographic mode, the table bucky is moved laterally in opposition to movement of the tube crane, with angulation keeping the tube aimed at the bucky. It is noted that the scan direction of the linear tomographic mode is the longitudinal direction.
In the art, though the conventional CT apparatus can perform an omni-angle scanning to clearly present tomographic images of the subject to be scanned, yet the accompanying radiation dose of the CT testing is too high. Hence, the effort to reduce the CT radiation dose, including effort to reduce the unnecessary radiation testing, is definitely crucial to the medical industry. It is noted that performing scan within a limited range could also be a resort to reduce radiation dose. By trading off between the limited information of the limited-angle scanning and the image quality, the choice of a correct scanning direction for different body portions seems to be one of the most important considerations. Though the current general purpose X-ray apparatus is capable of performing tomosynthesis scanning, yet the scanning direction thereof is still limited to the longitudinal direction. Obviously, possible merits from scanning along an additional traverse or other direction and with a omni-angle scanning range are still in vain for the conventional general purpose X-ray apparatus.